Two or more users want to share files, both with write privileges.
Create a group containing only those users, say, smith, jones, and ling:
Create the shared file in a directory writable by this group:
jones$ cd jones$ mkdir share jones$ chmod 2770 share jones$ chgrp friends share jones$ ls -ld share drwxrws--- 2 jones friends 4096 Apr 18 20:17 share/ jones$ cd share jones$ touch myfile jones$ chmod 660 myfile jones$ ls -l myfile -rw-rw---- 1 jones friends 0 Apr 18 20:18 myfile
Users smith and ling can now enter the directory and modify jones's file:
smith$ cd ~jones/share smith$ emacs myfile
smith, jones, and ling should consider setting their umasks so files they create are group writable, e.g.:
$ umask 007 $ touch newfile $ ls -l newfile -rw-rw---- 1 smith 0 Jul 17 23:09 newfile
The setgid bit on the directory (indicated by mode 2000 for chmod, or "s" in the output from ls -l) means that newly created files in the directory will be assigned the group of the directory. The applies to newly created subdirectories as well.
To enable this behavior for an entire filesystem, use the grpid mount option. This option can appear on the command line:
# mount -o grpid ...
or in /etc/fstab:
/dev/hdd3 /home ext2 rw,grpid 1 2
group(5), chmod(1), chgrp(1), umask(1).